Transmission spectroscopy is the widely used observational technique that exploits the planetary transit events with aim to characterize the exoplanetary atmospheres. While it already revealed atomic and molecular absorption, haze layers and thick clouds in about two dozen of objects, the literature presents some worthy mentioned discrepant results on individual targets. An example of controversial results in the literature is the case of HAT-P-12b, which I solved successfully during my studies. Intriguingly, two independent investigations from different groups, one with ground-based observations and one using the Hubble Space Telescope (HST), concluded to contradicting transmission spectra, interpreted from opaque clouds in the atmosphere to a Rayleigh scattering slope. The optical slope in this case is affected by the orbital parameters yielding to inconsistencies. I will present this degeneracy investigated for the entire parameter space with aim to ameliorate the quality of the scientific output.